You have told the mom no calls and that you won't be answering the phone and she shoudl leave a message - she needs to read the story of the boy who cried wolf - someday she is going to need to tell you something important and will get ignored. Set the phone to go to answering machine and turn the ringer off. A quick glance at the phone every now and then will let you know if anyone has called and you listen to the message and reply only if it is a real friend - most of my friends know not to call during the day so any calls I get are usually telemarketers or rogers concerned about how many phone services I dont' subscribe to that they offer.

Another option is to answer the phone and just tell her sorry your child is busy playing right now and can't come to the phone would you like to leave a message....!